This invention relates to orthodontics and, in particular, to a new and improved orthodontic arch wire, and orthodontic appliance employing the arch wire.
In the field of orthodontics, it is conventional to use an arch wire to correct irregularities and/or abnormalities in the relationship between various teeth. It is conventional to apply wire-receiving brackets to the teeth with the exception of the distal molars to which tubes are applied. A parabolic arch wire is inserted to the wire-receiving slots of the brackets on the teeth and the ends of the arch wire are inserted and received by the molar tubes. Ligature wires or o-rings are used to prevent the arch wire from slipping out of the slots in the brackets. When attached to the brackets, the wires under flexural and/or torsional stresses which, due to the resiliency of the wire, exert corrective tooth-moving forces on the teeth.
As explained in U.S. Pat. No. 4,900,251 to Andreasen, the contents of which are incorporated herein by reference, it was customary, when using stainless steel arch wires, to form bends in the distal ends thereof immediately at the distal ends of the molar tubes to prevent mesial-distal movement. However, as is well known in the art, it is now not uncommon to form arch wires of shape memory alloys, the physical properties of which resist the formation of an effective distal bend. While arch wires formed of shape memory alloys have a number of advantages, they suffer from the disadvantage that they are typically free to slide in a mesial-distal direction until one end protrudes excessively from a molar tube so it contacts and irritates the patient's mouth. This is due to the fact that the arch wires made of shape memory alloys have a relatively low coefficient of friction and cannot be effectively bent at their distal end.
The aforementioned U.S. Pat. No. 4,900,251 to Andreasen, as well as U.S. Pat. No. 4,892,479 to McKenna, the contents of which are incorporated herein by reference, each disclose an orthodontic arch wire provided at its mid-point or vertex with a bend or dimple shown to be substantially perpendicular to the principal plane of the arch wire. The bend impedes movement of the arch wire since the bend can not slip past the wire-receiving slots of the brackets mounted on the patient's central incisors because the dimple is measurably larger than the vertical clearance afforded by the bracket slots.
While the "vertical" bend disclosed in the McKenna and Andreasen patents can be effective in impeding mesial-distal slippage of the arch wire, arch wires having such a vertical bend suffer from a number of disadvantages. One such disadvantage is that such arch wires are aesthetically unsatisfactory since a bend outside the principal plane of the arch wire is very noticeable when the arch wire is installed in the patient's mouth.
Arch wires having a vertical bend are also disadvantageous in connection with treatment of a rotated central incisor. Specifically, in order to engage a rotated central incisor, a leg of the vertical bend must be distorted out of its original plane often leading to rotation of the bend out of its original vertical plane. The urge to correct the tooth rotation by the elastic return to the arch wire's original formation is compromised by the fact that the arch wire with the vertical bend is not able to exert the same degree of rotational effort in this situation. This is especially true in the cases of rotated lower central incisors (as can be attested by orthodontic clinicians who refuse to use such vertically dimpled arches in such cases).
As will be understood by those skilled in the art, arch wires of the type depicted in the Andreasen and McKenna patents, i.e., having a vertical bend, will also be disadvantageous in connection with arch shifting. Arch shifting may be necessary to correct a poor bite that is in malocclusion and requires the movement of all the patient's upper teeth to the patient's left and/or all of the lower teeth to the patient's right or vice versa. In this case, elastics supply the moving force and extend between hooks and distal tubes fastened to the upper and lower arches. As is known in the art, during arch shifting, the hooks frequently become wedged against a bracket. For this reason, the hooks are able to be loosened by the orthodontist and shifted as necessary. However, using an arch wire having a vertical bend, as disclosed in the Andreasen and McKenna patents, would also involve the disadvantage that the vertical bend, during arch shifting, will become wedged against one of the central incisor brackets, preventing further shifting and further correction.
Arch wires having a vertical bend are also more difficult to manufacture, thus increasing their costs.